Hydrogen Burning

Hydrogen burning is the fusion of 4 hydrogen nuclei (protons)
into a single helium nucleus (2 protons + 2 neutrons). The fusion
process takes place via a series of reactions. Exactly which reactions
take place in a given star depends on its mass, and therefore its
core temperature and density. In the Sun, this process is called
the proton-proton chain. In stars somewhat more massive than
the Sun, a different sequence of reactions, called the C-N-O cycle
(Carbon-Nitrogen-Oxygen), serves to fuse hydrogen into helium.